Non-ionic X-ray contrast agents constitute a very important class of pharmaceutical compounds produced in large quantities. 5-[N-(2,3-dihydroxypropyl)-acetamido]-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodo-isophthalamide (“iohexol”), 5-[N-(2-hydroxy-3-methoxypropyl)acetamido]-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodo-isophthalamide (“iopentol”) and 1,3-bis(acetamido)-N,N′-bis[3,5-bis(2,3-dihydroxypropyl-aminocarbonyl)-2,4,6-triiodophenyl]-2-hydroxypropane (“iodixanol”) are important examples of such compounds. They generally contain one or two triiodinated benzene rings.
For example, iodixanol, marketed under the trade name Visipaque®, is one of the most used agents in diagnostic X-ray procedures. It is produced in large quantities by GE Healthcare in Lindesnes, Norway. The industrial production of iodixanol involves a multistep chemical synthesis as shown in Scheme 1 below. See also U.S. Pat. No. 6,974,882. To reduce the cost of the final product, it is critical to optimize each synthetic step. Even a small improvement in reaction design can lead to significant savings in a large scale production.

5-acetamido-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodoisophthalamide (“Compound A”) is a key intermediate in both the industrial scale synthesis of such non-ionic X-ray contrast agents. Compound A is prepared by the acetylation of 5-amino-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodoisophthalamide (Compound B). The acetylation is achieved by using a mixture of acetic anhydride and acetic acid as the acetylating reagent. However, upon acetylation, not only is Compound A produced but several by-products are formed as well.
Thus there exists a need in the art for an acetylation process that can produce Compound A with a lower level of by-products; thus increasing both purity and yield of Compound A. Such an acetylation process must not only be able to be performed on a laboratory scale but also on an industrial scale. The instant invention, as described below, answers such a need.